text:
Can I get a third-class medical if I have sleep apnea?
Some year-and-a-half ago my physician recommended that I take a sleep study. Some preliminary exam (overnight finger-carried monitor) indicated that a few times per night my oxygen level dips below 91% (and one time below 81%). He indicated that this might be caused by sleep apnea. I ignored that recommendation as my sleeping had improved and I didn't want to spend money and time on the full-blown sleep study. Six months after that I started my PPL training. As it was not an issue, I didn't even think about it when I got my 3rd Class med certificate.
My soon-to-be-ex wife had recently found my old correspondence with my doctor and, out of pure vindictiveness, denounced me to the FAA. I received a letter giving me 60 days for addressing this issue. Here are my questions:

Does anyone at this forum has a mild case of sleep apnea and is still flying (i.e. has a valid third class med)?
Is the dip in the oxygen intake an 'sure proof' of sleep apnea?
Are there any other types of SA than Obstructive SA that are not viewed as dangerous by the authorities (the regs specifically mention OSA)?
Is there any recommended treatment that may still make the med cert possible even if I have a mild case of SA?

Did you read the FAA guidance on OSA/Sleep Apnea? You'll need to get a Special Issuance to continue flying, which isn't impossible, but you need to be working with an AME.

The short answer to your question is yes, you can get a third-class medical with sleep apnea in some cases:

If your OSA is treatable, you can maintain your airman medical
  certificate and continue to enjoy your aviation career.

Everything else in your question is very personal and specific, and the only way to get an answer for your individual case is to consult an AME, preferably one who's handled similar cases before.

I know two professional pilots who's medicals were revoked but got them back after mastering sleeping using a CPAP machine and demonstrating compliance with the requirements. Their Apnea diagnosis was based on the number of breathing-stop events measured by a breathing monitor (not more than once or twice a night if I recall). The problem is mostly that Apnea events cause deprivation of REM sleep and result in severe fatigue, while the person is unaware of the constant sleep interruptions because they don't fully wake up and think they are sleeping fine.
One of the pilots is a good friend and went to the doctor because he thought he was sleeping ok but was dog tired all the time and couldn't understand why.  He had to sleep with a monitor and when it showed numerous events he was grounded immediately (about a year ago) but is now flying again after mastering the CPAP.  He said the sheer joy of getting a good night's sleep made it easy to put up with the CPAP mask once he got used to it.

Thank you very much for your very informative answers. This very painful for me not only because of the impact on flying, but on the more personal level (the vindictive attitude on the part of the person who denounced me). I'm rarely tired, or sleepy during the day, so I hope, this all end well.


text:
Do tower-controlled helipads have a standard naming convention?
Just wondering if anyone knows, or if it just depends on the airport.
Edit: This question is different than How does the FAA determine which format of location identifier to assign to an airport?.
My question is how the tower names helipads under its control at an airport, not how the FAA assigns an identifier to an airport as a whole. Runways use the runway heading. How are helipads named?

Do you have an example? Usually helipads fall under "parking" and the pilot would just tell the tower what ramp they wanted to go to. After that whoever controls the ramp (like an FBO or other operator) would direct the helicopter to a specific pad.

Stand-alone helipads are naked just like any other airport. So I’m VTC as dupe of How does the FAA determine which format of location identifier to assign to an airport?

Possible duplicate of How does the FAA determine which format of location identifier to assign to an airport?

@RonBeyer AFAIK, at LAX, the helipads at the south ramp are called "South Pads". Those are controlled by the tower. My question is about those ones. I think the helipads you're talking about are behind the non-movement area boundary (and thus are not controlled by the tower).

If an aerodrome has more than one helipad, they are just named "somehow".
In Bückeburg (Germany) we have one "helipad" and one "VIP Helipad" or sometimes named just "VIP spot". On radio comms they are named the same.
"HeliXXX cleared to land Helipad", or "HeliXXX cleared to land VIP Spot".
A police school in germany named its helipads Helipad 1, Helipad 2, Helipad 3 ...

Normally helipads are allocated names depending on what taxiways they are adjacent to. For example, Helipad Alpha would be off or near taxiway Alpha. Helipad Charlie would be off of Taxiway Charlie. Results may differ depending on the airport/heliport layout, but this is how the normal naming system works.

Do you have a source for this? Or could you add an example?

Looking in to this further it looks like the FAA differs slightly from the ICAO standard. Helipad's are simply named H1, H2, H3, etc based on the number of helipads located at an airport.
https://www.faa.gov/documentLibrary/media/Advisory_Circular/150_5390_2c.pdf

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text:
How to say the name of this approach clearance: COPTER RNAV (GPS) 027°
JFK has this approach: https://aeronav.faa.gov/d-tpp/2006/00610COPTERR027.PDF
How is this approach said on frequency? Is it the "copter RNAV zero two seven approach" or the "copter RNAV zero two seven degree approach" or the "copter RNAV heading zero two seven approach" or something else?

Ultimately, i don't think it really matters, as long as ATC can understand what you are saying.

Degrees is never used on frequency. Could either be temperature or heading so you don't use it. Then the word heading is not required as the control knows what your talking about. The rule is keep it as short and clear as possible. So the first solution would be my answer.

Based on the 7110.65 2–1–17h, "degrees" is never used. Based on Note 9 under 4–8–1a, the text in paranthesis is omitted. So, ATC would say:

Cleared Copter RNAV zero two seven Approach

or

Cleared Copter RNAV heading zero two seven Approach

If you were requesting it, you'd say "Requesting Copter RNAV zero two seven approach."
However, since it is the only Copter approach at JFK, you could probably just request "the copter approach" and they'd know exactly what you're talking about. But ATC is required to use the phraseology as indicated in the Order linked above.

"Copter RNAV  zero two seven approach"

Could you add a source? That could also be personal experience with this approach or similar ones. In that case, could you add some more details?

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text:
Are helicopters expected to follow the lateral guidance of taxiways in a hover taxi or air taxi?
If a helicopter is asked by ATC to hover taxi or air taxi, are they still expected to follow the lateral guidance of taxiways on the ground, or do they proceed "as the crow flies", direct to their destination?

How can one hover and taxi? One negates the other

Yes, unless given an explicit clearance to hover taxi direct to a location on an airfield.

The question is phrased as “which of options A or B applies”? “Yes” is ambiguous.


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"Report a 3 mile right base" - where is it measured from?
At which position should you report to ATC?

How long a base leg would you actually be flying in the "position 3" case?

This IS a duplicate...

It's neither of the above, it's a base leg which will converge onto the final approach leg 3 miles out from the runway threshold. In other words when you turn from base onto final you'll be on a 3 mile final.

Do you report when established on the base that will intercept final at 3 miles?

I've never been told to report on a 3 mile base before, but if I was I would report when established on the base leg as the key part is to report on base.

How is that different from "Report a 3 mile final"?  If you're turning base-to-final, or just on final, you're at the same position, 3 miles from the threshold, just on a different heading.

The difference is that the controller wants to know when you enter the base leg, not when you are on final @abelenky. Maybe it's important for separation. I'm not convinced this is standard phraseology.

I like this answer.  I'd like even more to see if there is a source.  PC glossary?

A "3-mile base" means a base to 3-mile final, with downwind being at the normal distance from the runway; it's named that because it's the base leg that moves.
All of your positions are based on the idea of a 3-mile base to (normal) final, which would mean the downwind (not base) has moved.  If that's what they wanted, you would get an instruction to simply "enter right base", regardless of how far away that may start, and possibly some other reporting instruction if they're non-radar.
Also, position 1 doesn't really make sense.  If you called in north or northeast of the field, they would tell you to "enter downwind", so position 1 shouldn't happen.  If you were already in the pattern and they wanted you on a 3-mile final, they would tell you to "extend downwind" and either tell you 3 miles (or a landmark 3 miles away) specifically or, more likely, tell you what traffic to follow.
If you called in east or southeast of the field, tower would assume you're in position 2 or 3, not position 1, because any sensible inbound caller would have their nose pointed either at the field generally or at whichever leg of the pattern makes sense given the runway in use (i.e. wind) and their position.


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What is the top altitude of the class delta KEDC? It looks like it's not depicted on the VFR sectional chart
Can someone tell me what I'm missing? I usually see a [-#] that denotes the airspace above a class delta. I believe KEDC (Austin Executive) is a class delta based on the chart supplement.
Attached is the VFR sectional from vfrmap.com:

This looks odd to me.
The supplement (3 Jan 2019) does indicate class D operations:

AIRSPACE: CLASS D svc 1200–0400Z‡ other times CLASS E.

But the current (August 2018) Airspace Designations and Reporting Points doesn't list class D airspace for it, only class E.  This matches the sectional shown.
When I look at another part-time airport (like Drake Field/FYV), the airspace is listed both as class D and class E as expected, and the height of the class D is given.
Perhaps something needs to be updated?
UPDATE
Yes, looks like the control tower opened at KEDC Feb 2018.  It should eventually be charted.  Until then, NOTAMS...

KEDC NOTAM:
!EDC 01/001 EDC AIRSPACE CLASS G SFC AREA CHANGED TO CLASS
  D 1901031200-PERM

And the update section of the supplement.

8 Nov 2018 No Major Changes.
3 Jan 2019 Add AUSTIN, TX Class D: That airspace extending upward from the
  surface to and including 3,000 feet MSL within a 4.1-mile radius of
  Austin Executive Airport, excluding the Austin Class C airspace. This
  Class D airspace area is effective during the specific dates and times
  established in advance by a Notice to Airmen. Revise AUSTIN, TX Class
  E: That airspace extending upward from 700 feet above the surface
  within a 14-mile radius of 30º17´55″ N, 97º42´06″W ....

I don't know why the airspace change is marked as Jan 2019 when the tower opened in Feb.

Sectionals only get updated every 24 weeks. The San Antonio sectional here was published Nov 8 2018 and is due for update on Apr 25 2019.  Changes between chart updates are included in the Chart supplement every 28 days. The change shows in the Supplement but not the chart,so I would assume the change occurred after the freeze date for the Nov 2018 chart changes. It should show up on the Apr 25 2019 Sectional.

If this were a recent change that's simply not yet charted, I would expect it to be mentioned in the A/FD updates area.  I don't see any mention there.  https://aeronav.faa.gov/afd/03jan2019/SC_rear_03jan2019.pdf.

It's listed as an add on Page 422. Since it says effective by NOTAM, it may not be charted or the NOTAM may be the interim 'solution' waiting for the chart update.
AIRSPACE
8 Nov 2018 No Major Changes.
3 Jan 2019 Add AUSTIN, TX Class D: That airspace extending upward from the surface to and including 3,000 feet MSL within a 4.1-mile radius of Austin Executive Airport, excluding the Austin Class C airspace. This Class D airspace area is effective during the specific dates and times established in advance by a Notice to Airmen.

Man, I looked at the right document, but that didn't help me read it.  I must have read the wrong page and not realized it.  That's exactly what I wanted to see.

This interested me enough to actually call the tower in order to hear some kind of explanation from a tower controller. The only explanation I got was that the ceiling, when active class D, was 3000 feet. I asked how anyone would know this and she told me that it was "published." I'm not sure where it's published but the conclusion we both came to is that the charts coming out in the next cycle (in a few days) should have the 3000 ft ceiling denotation.

I guess "published" means in the supplement and the NOTAM.

I don't think the controller even really knew too much about where to find this information otherwise they could have just said check the NOTAMs.


text:
Is the pitch an attitude indicator displays in relation to the ground, or is it in relation to the current aircraft bank?
In the old analog, vacuum powered attitude indicators, does the displayed pitch axis indicate the pitch in relation to the current bank angle, or in relation to the ground (true level)?
To put it another way: if an aircraft was flight in a very steep turn (lets say 70 degrees) would the attitude pitch indication be more affected by rudder inputs or yoke forward/back inputs?
If a stunt plane is flying in knife edge flight with the wings rolled to a bank angle of 90 degrees, does pushing (or releasing) the rudder change the Attitude Indicator pitch indication at all? Or would only pulling or pushing on the yoke change the Attitude Indicators pitch indication?

Not following the question. Starting with a bank of 70 degrees and a pitch of X... are you asking which of rudder or back pressure would most affect the pitch of the aircraft? Or are you seeking a way in which the instrument indication is affected apart from the attitude of its aircraft?

Prob question could be best answered with a picture of the instrument face in an a/c at 45 or 60 degree bank, with a line drawn showing what two points on the face, the distance between which represents the aircraft's pitch attitude.  Do we draw the line straight up and down in the reference frame of the cockpit, or do we draw the line perpendicular to the horizon bar?

@quietflyer If the former, the value for "pitch" would get larger & larger without bound as the bank angle approaches 90 degrees, and would be undefined at a bank angle of exactly 90 degrees, since the line perpendicular to the floor of the cockpit then doesn't intersect the horizon. Can't see how such a convention could ever possibly be useful; the pitch angle there would change with every roll input.

Also related --https://aviation.stackexchange.com/a/16538/34686  key sentence -- The rotations are applied in backwards order, first yaw (ψ), then pitch (θ) and finally roll (ϕ).  Which implies to me that rolling shouldn't affect the "pitch angle" , but yawing would.  So yawing, but not rolling, would affect the displayed pitch attitude-- if the instrument is marked correctly.  Not 100% sure about all that but...

@RalphJ I'm trying to understand what the instrument is telling me at the edges of the envelope of values it can display. I have a pretty good concept of what the control inputs will do to how the plane is flying, and only talk about control inputs as a way to describe how the plane (and thus instrument) is being rotated in space. I could experimentally answer my question w/ a AI out of the plane w/ a vacuum source (spun up while level/still). I would pick it up, rotate it so it showed 90 deg bank with a 0 pitch, and then see which other axis I have to rotate it in to get a pitch indication.

@Azendale  -- that would be the yaw axis. As per my comment immediately above.  From a 90 deg bank with 0 pitch, you could yaw the nose up to a vertical climb if you wanted: 90 degrees pitch.  Unless (as your last comment suggests) you are interested in flight attitudes where the instrument capabilities are limited in some way?  Not sure what you mean by "edges of the envelope", why should the envelope have any edges at all?  Unless you are talking about "gimbal lock", "tumbling", etc?

@Azendale  The answer provided by me was edited with an additional illustration.  An added comment, also provided, may give additional clarity.  Think about what you are seeing from within the airplane, and the instrument's measurement frame of reference outside of the airplane.  This video by Ryan Anderson may be helpful. link   There is another video about the mechanics of the attitude indicator on Ryan Anderson's youtube site.  Hopefully this has been helpful...

@quietflyer The edges of the envelope comment was a realization that when I start talking about a 90 degree bank, I might hit a limit on some mechanical device, resulting in tumbling or gimbal lock. But a 90 degree bank is helpful in explaining the question because it makes the axes in reference to the aircraft that used to align to axes in reference to the ground become completely orthogonal -- other than the one we rotate 90 degrees around.

@Azendale -- clearly implicit in the above comments, is the idea that "degrees of rotation about the aircraft's pitch axis" and "degrees of change in the aircraft's pitch "angle" aka pitch "attitude"" are often very different quantities.  This ASA question is somewhat relevant: https://aviation.stackexchange.com/q/97987/34686.  Note that my own answer (to my own question) turned it into a navigational problem!

@Azendale  Are we good, here?  A link to the attitude instrument simulator at Embry-Riddle University is provided in my comment addressing your additional question (for clarification), below.  An interesting question.  Thanks for asking!

@ThomasPerry Yes, it has helped. Especially the simulator. I now trust the AI more, knowing what it is saying in a very steep turn. I've done some trig but was having a hard time following what the trig in them was implying -- would you be upset if I accept your answer and then (if I can either find or draw better ones) edited it to try to make it show rather than use trig to explain? I will say I'm also realizing I was having trouble with the definition of "pitch".

@ThomasPerry the last picture of a steep turn helped, though I think I would have understood faster if the triangle was labeled "indicated pitch" and we left out a sub T and added an arc near the bank angle label.

In the old analog, vacuum powered attitude indicators, does the displayed pitch axis indicate the pitch in relation to the current bank angle, or in relation to the ground (true level)?

Yes.
For the illustration given below -

the analogue for the instrument of interest is shown below -  Note that pitch is relative to the horizon and measured in the vertical plane aligned with the aircraft center of gravity and tangent to the flight path.  Note that $\alpha_T$ is greater than the indicated pitch.  Follow carefully the elements indicated in these illustrations...  "Flight path" is sensed by the pilot along the direction of lift for the path of flight in the banked turn.  An actual example in flight for this analogue is shown, below.

...in a steep turn...would the attitude pitch indication be more affected by rudder inputs or yoke forward/back inputs?

Quite clearly, the rudder can change the pitch in a steep bank.  The influence of the rudder would be, for instance, to yaw the nose farther above the horizon, or farther below.

If a stunt plane is flying in knife edge flight with the wings rolled to a bank angle of 90 degrees, does pushing (or releasing) the rudder change the Attitude Indicator pitch indication at all?

Yes, if the instrument is working correctly, yawing the aircraft in a 90 deg bank would be indicated by a change in corresponding pitch either above the horizon, or below.

This answer is in a helpful direction. The drawing of the airplane and a new geometric plane helps. Diagram is a bit busy so I'm having a hard time perceiving it in 3d space.  But my core question boils down to, I think, does the distance labeled "arcsin(sin a sub t cos o)" on the instrument face measure a rotation in the geometric plane drawn/shown, or only in the external vertical geometric plane?

@Azendale  an additional illustration was added that may help. All of the elements in the illustrations are shown in the last one, the actual in-flight example.  Note that pitch is a seemingly indirect measurement of the instrument which is, however, actually measured directly.  I know, this seems complicated...  but the instrument solves this presenting the analogue of the measurement as a direct, indicated reading for pitch in actual degrees.  Note the alignments...  The noted angles are what the instrument is measuring, namely pitch $arcsin(sin\alpha_T cos\phi)$, & angle of bank $\phi$.

Ok, so, if I understand right: If I go into a 90 degree bank steep turn, no matter how hard I pull the yoke/stick ("up" elevator), I will get no change in pitch.  But if I push the rudder to the stop, causing yaw (defined relative to the aircraft) I will cause a change in AI indicated pitch. Correct?

@Azendale Yes, this is a paradox.  $\alpha_T$ is pitch relative to the flight path.  In a 90 deg. banked turn, there is no pitch relative to the horizon, only pitch relative to the flight path.   Nevertheless, apparently so, in a 90 deg. bank rudder induced yaw bringing the nose above the horizon, or below, would be indicated by corresponding pitch above the horizon, or below.  Embry-Riddle University has an excellent pitch-attitude indicator simulation link where you can simulate exactly this situation.

(It seems these spherical geometry problems may be solved in more than one way-- building the equations upon the assumption of a fully coordinated turn is one valid way but not the only way-- other ways are independent of knowing anything about the flight path or the state of coordination/uncoordination-- came to light here https://aviation.stackexchange.com/q/97987/34686 -- see esp footnote 3 of my answer-- anyway third diagram is improved now that "pitch" is clearly labelled, thanks -- )

(Observation pertains to TP's comment immed. above and also to understanding the situation of a stunt plane flying in a steady-state constant-altitude non-coordinated knife-edge (90-deg bank) turn with nose held above horizon with top rudder -- )


text:
Could robots take over pilots' jobs and fly planes - if they can, then how?
As technology is advancing, could robots take over pilots' jobs? If so, how might they accomplish this?

Sure, someday. Then we just need to wait for teleporters and we'll take planes away from robots starting the robot revolution. This is how the apocalypse starts.

Theres a new Video from an Aviation Channel I like: https://www.youtube.com/watch?v=03HOFVS3r70 He wants to tune the Piper to start, taxi and fly autonomically.

It's already in the works.  Fully autonomous unmanned combat air vehicles are no less than 10 years away.  I'd say that the major airlines will follow in 10-15 years, most likely starting with the elimination of the First Officer with fully automated functions.  Similar systems will be introduced, and most likely before the airlines on corporate jets.
It will be fought tooth and nail by the pilot unions, but sooner or later the algorithms these AI pilots use will become increasingly sophisticated to the point that there will be another incident similar to that faced by US Airways 1549 where an AI is under control and makes a decision saving the crew and passengers where 97% of all human pilots who, when faced with an analogous situation, failed resulting in a catastrophic accident.  Thus public confidence will be bolstered in these systems and public opinion will sway in their favor - even demanding these systems will be on board.  All that's required at that point is flights operated by AI are 40% more efficient and 63% more likely to be on time without interruptions of service due to weather, crew fatigue, etc.  At that point the first experimental, totally unmanned commercial flights will happen, probably after the corporate jets lead the way.
Military wise a similar thing is bound to happen as ACM algorithms improve, making super maneuverable drones which are capable of defeating Red Flag instructor pilots 97% of the time and more and more years of confidence built on the AI systems both in peacetime exercises and in conflicts will finally eliminate the manned fighters and bombers.
Perhaps the last vestiges of manned flying will be in general aviation and remote bush flying for commercial reasons (the revenge of GA?), but even these will slowly be eliminated as AI controlled aircraft and infrastructure capable of allowing off field landings and takeoffs, even in the most remote or difficult places come online.

Thanks, but how can robot planes fight head to head with human piloted planes.  The sky is 3D, up down left right, missiles guns and weather changes, stall, observations and more.

Depends on how sophisticated one can write the algorithms to control this.  If you watch that NOVA program I referenced, you will see that such things are under development.  In some ways an AI could have a major advantage over piloted aircraft in a merge as they have far more precision in their timing in maneuvers and often the first turning maneuvers can mean the difference between victory and defeat.

@DanielChung A robotic fighter aircraft going head to head with a manned aircraft has a significant advantage in that it can maneuver at much higher G loads than a human pilot can withstand.

This answer makes some very bold claims without much evidence, like that unmanned pilot technology will likely be deployed on airliners in 20-25 years, that a pivotal near miss will hasten the adoption, and that AI will eventually replace virtually all pilots. If you want this to be the accepted answer, please provide evidence or citations for such extraordinary claims.  For expected levels of evidence see http://meta.aviation.stackexchange.com/questions/333/what-is-the-standard-of-evidence-required-in-answers/

But @Cody, what evidence do you expect about future events? It's bound to be an opinion. The question is hypothetical: could they? and this answer is: they could, perhaps, and even likely.

A lot of numbers, without proves. You claim that pilots are causing 63% delays and weather as example? That doesn't make sense. Furthermore your missing the wishes of the passengers and regulations. US Airways 1549 is a good example for a situation where an computer will likely rely on pure failsafe-code or even don't believe on a two-engine-out-scenario without a technial reason. And you cannot predict what an AI would do, including sacrifice passengers for habitants or vice versa. Good example for what an pilot is good for is also LH 1829: http://avherald.com/h?article=47d74074/0000&opt=0

@Zeus If it were only possible to answer this question with speculation and not fact-supported predictions the question should be closed as "opinion based".  This is a fact-based site, not a discussion board. As such even futuristic claims should be backed with information like advances being made in similar industries, advances in the last decade towards the prediction, industry leaders' opinion, and notable R&D projects.  This answer could cite any of those (besides an unexplained link to a NOVA video) and so would be much more convincing.

Until some accidents due to 'connection to server failed', possibly yes...

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text:
Do modern aircraft have sensors to determine weight of the aircraft?
Do ALL modern aircraft, particulary airliners and private jets, have weight sensors on all landing gear to determine take off and landing weight in real time so as to determine take off speed and approximate landing distance?  If so were are they installed and if not why not? Are there any attempts to install these on older jet airliners?

Related: How does a commercial airliner measure its weight/mass? and also How do airlines handle uncertainty in passenger weight for weight and balance calculations?

Related: Do some airplanes have weighing scales on the landing gear?

No, the 737 does not have such sensors installed because it is less expense to compute the zero-fuel weight (and thus takeoff and enroute and landing weights, by adding fuel on board) by taking the known aircraft empty weight and adding to that the weights of the people and things that are on board.
I've never heard of any attempt to install any such sensors on a 737. Some FMC software can use in-flight performance data to estimate gross weight and alert the pilots if that estimate varies too much from the computed weight, but that is an entirely different approach.

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text:
Why don't plane wings start at the front of a plane and end at the back?

This is what some people think the SR 72 will look like.
If a plane wing was attached to the whole fuselage, couldn't it have a shorter wing span? If it had a shorter wingspan it would be less of a lever so could be less strong and therefore lighter. I don't mean a blended wing.

that's called "flying wing". Related: https://aviation.stackexchange.com/q/17119/1467 and https://aviation.stackexchange.com/q/2252/1467

What you say is true for supersonic designs. At subsonic speed it helps to stretch the wings out a bit to reduce drag. Making their chord deeper than necessary creates too much wetted surface, which results in excessive friction drag.

You are right, a short wing span is a blessing for the structural design and the wing will be nice and light. But since you need a certain wing area to keep the aircraft in the air, the wing will have a long chord as well, and that won't be good for aerodynamic resistance.
For subsonic aircraft, you want the longest, narrowest wing possible: the lift created by the wing is caused by air accelerating downwards. The amount of air streaming in is a function of the span, so a long span accelerates a lot of air downwards only a little bit; a short span wing has to accelerate much less air much more. That creates induced drag: the horizontal component of the backwards tilted lift vector.

Another source of drag is the tip vortex, where air 'flips over' from high pressure below to low pressure on top of the wing. So for subsonic aircraft, the longest, narrowest wing is the most beneficial from an aerodynamics standpoint. It will add constructional weight and therefore increase weight induced drag, but will compensate for it by a reduction in aerodynamic induced drag.
Supersonic aircraft have an additional source of drag: shock waves. The air gets no warning that an aircraft appraches. The nose arrives first, and a shock wave streams off of the nose in the form of a cone. Inside the cone, the air has lower velocity - if the wing can stay within the cone, it creates much less supersonic drag.
Image source
The higher the supersonic speed, the more the shock cone bends backwards, and the widest bit of the wing needs to be more towards aft to remain within it. So for supersonic aircraft, the delta wings that you draw are in use. They create a relatively huge drag when flying subsonic though.

One of the issues with flying wings / blended wing bodies is ground servicing. All kinds of people or vehicles require all kinds of access to all kinds of systems which is more difficult to arrange (not impossible, but more complex) than the ubiquitous tube-and-wing design.

What I'm trying to describe is not a BWB or flying wing. In those designs the fuselage is surrounded by wings. I mean if you made a normal (747 for example) jet have it's wing start at the front end at the back and maintain its thickness therefore engines etc would be equally accessible. The one harder thing would be loading but that could be done.

Consider also skin effect. In your scenario, the larger wing surface would create more drag than a typical configuration.
Edit, ok I see what you are saying now. And yes, your assumptions are correct.
Your question seems to be, why isn't this more common place?
If that's correct, the answer is stability and controllability. These designs are inherently less stable than what you would consider "conventional"
There are plenty of examples of aircraft that have attempted to overcome the hurdles, the verhees delta comes to mind.

You end up with the Chance-vought V173.
Aircraft design is a science of compromises. The V173 is an example of optimizing one advantage over all others, and departing too far from a balanced optimum.
This aircraft actually flew very well and met all its design criteria. Unfortunately these criteria were strongly rooted in the propeller era and did not scale into the jet age.


text:
Does taxiing in London City (LCY) happen on the runway?
I recently had a ride to LCY (London City). Awesome, by the way. They have only one runway (09/27) and no apparent taxiways.

(Source: Google Maps)
After landing, how do the aircraft get from the runway to the terminal? Do they just perform a U-turn on the (end of the) the runway and taxi back to the terminals?

Nomenclature: taxiing down the runway in the opposite direction of takeoffs/landings is known as a back-taxi.

Probably, since there doesn’t appear to be any other option.  Since you were just there though, weren’t you able to observe this directly yourself?  Or I guess you didn’t think about it at the time and only noticed when looking at the overhead image later...

I don't understand your question. There is no possible alternative to taxiing back along the runway. What else could you possibly even imagine as an alternative?

Why the down vote? Question whose answer is obvious to many may not be obvious to others.

This question isn't exactly a dupe, but it gives a lot of relevant information

@Jimmy The asker is aware that taxiing down the runway is a possibility. There are no other conceivable possibilities.

The photo clearly shows a short bit of taxiway south of the runway 27 threshold. There seems to be enough of it that at busy times you could convoy three or four departing aircraft out to it along the runway at once, and have the latter of them wait out to the side while the ones in front take off. And close-together arrivals on runway 09 could do the opposite.

@DavidRicherby Certainly there's an alternative to back taxiing. If you're landing on 09, when you get to the 27 end, you pull off on the little taxiway there, hop on an awaiting barge and float back to the terminal area. Obvious! ;)

@DavidRicherby There are lots of conceivable alternatives. Maybe landing is only permitted on 27, or maybe aircraft are required to taxi in the grass, or maybe this airport is only open to aircraft that are capable of vertical takeoff. It's not necessarily obvious to everyone that aircraft backtaxi here on a regular basis.

Taxiing back to the other end is a "backtrack".  At a controlled airport like that, the backtrack would need a clearance.
When you land at London City, you will roll out and turn off onto that little turnaround bay, where you are "clear of the active" and you would then need a backtrack clearance to return to the other end.
It would depend on the traffic level.  If you landed and someone was right behind you, you might need to wait for the other a/c to roll past and then you'd get a backtrack clearance to pull back out onto the runway to taxi to the other end.  If it's not busy, the tower controller may give you a backtrack clearance as part of your landing clearance so you can land, turn around and proceed straight back.  The controller can't give a landing clearance to anyone else until you are clear.
In any case, the main thing is that you can't proceed back without permission from somebody.
Note that London City is now controlled offsite using special cameras and sensors, but it works the same way.

Your answer seems to contradict the quote from UK AIP posted by ymb1, which states that after landing runway 09 aircraft should backtrack immediately, NOT roll out to the turnaround bay.


text:
Are airliners searched after each flight?
When a commercial airplane lands at an airport, is it always searched for suspicious objects?

They are always cleaned. But more for hygienic and lost&found reasons than bomb threats.

At the end of the Airplane's day (It's last flight for the day) It's the mechanics and the cleaners' turn to be on the plane.
The mechanics start out by performing the daily check or overnight check, it's like a routine checklist to make sure that your airplane is well maintained and ready for another day of flying. They also look at the Cabin Log books of the aircraft for any maintenance issue the flight crew has encountered during the day.
These issues could be anything:
ex:
"Seat at Row 28 won't recline properly"
or
"Sink is clogged"
and while the mechanics are resolving those cabin issues along with their daily checks, the cleaners also start their work. Cleaning all the seats, galleys and lavatories. They are not particularly looking for bombs, but they make sure to report any lost items they've found during cleaning.
Almost the whole cabin is cleaned up and inspected. So if anything is left, it will be found. Even the life jackets underneath your seats are accounted for, so if anyone decided to leave something under there, the mechanics would spot it.


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Does ETOPS apply to Part 125 operations?
Does ETOPS apply to aircraft operated under Part 125? I see ETOPS mentioned in Part 121 Appendix P and Part 25 Appendix K, but it would appear that ETOPS wouldn't apply to Part 125 operated aircraft?

Yes I do mean Part 125 Operations, and edited the question to clarify. However the question was further edited by the moderator (I assume) and doesn't show my edits.


text:
Instructing your Kids
Can anyone cite any regulation that would prevent my child (not infant) from logging time for a private pilots license (i.e. 12 years from now when they are 16 for a solo, 13 years for a license when they are 17)?
Background: I have children that I would like to start instructing and having them log flight training when they fly with me. I am VFR/IFR and medically current, and have a current FAA flight instructors license, therefore my question doesn't involve my own qualifications. Just if there are any known restrictions against the kid logging flight time. To clarify I don't mean both kids logging time on the same flight, just one kid.
I have found none in the CFRs (e.g. 61).

So this is a 4 year old you want to instruct? I hope there is regulation against that.

So when they go for a job interview now they have to explain why it took them 13 years to earn a PPL.

If your child is going to be around airplanes and flying with you for the next 12 years, they're not going to need to log time from back when they are 4 years old. They will have plenty of logged time later in life.
I would suggest that you pick an age, say at least 8 or 10, where your children can start to fill in log books of their own. If they keep an interest in aviation until that time, getting their own log book that they can fill in themselves will be a big step for them to look forward to.


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How does the lift coefficient of an A380 change when flaps are extended?
I would like to know the lift coefficient with 10 degrees of flaps extended.

Why do you want to know this? Under which circumstances; in a windtunnel where everything else remains the same, or during flight where extending flaps happens together with many other parameters changing?

As mostly when questions are vague, the answer is: It depends.
If nothing else changes, the nominal lift coefficient stays the same. A coefficient is referenced to an area, and for aircraft this is the reference area which is derived from the wing area. Regardless of flap settings, the reference area stays fixed, and so does the lift coefficient because neither the mass nor the speed of the aircraft have changed when flaps have been set.
In reality, the aircraft will slow down when flaps are extended, and it can afford to do so because drag is increased and stall speed lowered. Now, with the lower flight speed, the lift coefficient will go up in proportion to the ratio of the speeds before and after squared:
$$c_{L_{flaps}} = c_{L_{clean}}\cdot\frac{v_{clean}^2}{v_{flaps}^2}$$
Again, this assumes the same reference area.
In a wind tunnel where the aircraft model is fixed to a sting and the angle of attack is controlled, setting flaps will increase the lift coefficient by the difference in zero-lift angles of the aircraft with and without flaps, multiplied by the lift curve slope of the aircraft.
$$c_{L_{flaps}} = c_{L_{clean}}+ (\alpha_{0_{clean}} - \alpha_{0_{flaps}})\cdot c_{L_{\alpha}}$$
Note that the lift curve slope increases when setting flaps increases the effective wing area. Again, all is referenced to the unchanged reference area.

With extended flaps, the chord length of the airfoil increases, increasing area of the wing itself. This would decrease the lift coefficient of the wing, thanks to this formula from NASA:
Cl = L / (A * .5 * r * V^2)
Hope I could help, at least partially.

When an aircraft flap is deployed, it increases the effective camber of the wing airfoil. Thus, the zero lift angle, i.e., AoA at zero lift decreases as a result the cl-a curve shifts to the left..
As you would know, a cambered airfoil has higher cl than a symmetric airfoil. It is the same concept here. As the camber increases, so does the cl.
To calculate the change wrt flap deflection you need more information. Either you could do this physically on a model of A380 or you could try to plot the curve if you have aircraft aerodynamic data.

This would be true if nothing else changed, including AoA. In this case the aircraft would start climbing (initially). But in all practical situations we'd assume that we want to maintain steady flight, and this means that we want to keep lift the same. Which means: either cl must stay the same, by reducing AoA, or we slow down and choose the appropriate cl (and AoA), which can be greater or smaller depending on the chosen airspeed.


text:
What is the absolute ceiling of a 737-200?
We're trying to find the official absolute ceiling for the 737-200 for a school project.  So far have only been able to maximum and service ceilings for this craft. Can anyone point us to the correct information for absolute ceiling?  Thanks!

That will depend on weight, including fuel load

While it depends on weight, there is a maximum anyway when the aircraft is at its minimum possible weight.

I don't believe §25 certified transport category aircraft will typically have a published absolute ceiling. Try looking for a Maximum Certified Operating Altitude. In actual practice, maximum altitude attainable will depend on various factors such as air temperature, aircraft loading, and engine limits.

@JWalters: Why wouldn't they?

@sean That may be best addressed with a new Q&A. Short answer is that the absolute ceiling is typically well above the certified operating altitude and is therefore not pertinent. Look at various TCDS for examples of the limiting altitudes. One primary limiting factor is time for descent to safe altitude in the case of a pressurization emergency.

Find the pertinent  performance charts for Vx (Max climb angle)and Vy (Max climb rate). You may have to extrapolate, but the altitude where Vx and Vy converge is the Absolute ceiling.

If the OP cannot find the maximum ceiling value, it's unlikely they can find Vx / Vy charts.

This is a jet powered, §25 certified aircraft. There will not be a published Vy or Vx.


text:
How can someone without a pilot's license arrange an initial test flight for a homebuilt aircraft?
I think this has more to do with culture and expectations than actual rules, so maybe this might attract opinionated answers, but I still want to know what people think:
If a person were to construct their own homebuilt, like a Kitfox or RV, without first obtaining their PPL, how can they go about completing the 25 hour initial test phase required for the new aircraft?
This encompasses a subset of questions:

Is it customary to pay an experienced pilot to do it, acknowledging the risk involved with the activity?
Or is free use of an aircraft and loggable time/experience compensation enough?
Is it generally hard to find someone willing to do this for you (maybe do this with you is a better phrase since you will be part of the ground crew)?

I assume that this is under US/FAA rules?

@RonBeyer yup, added that.

Aside from being a bad idea - 1. Until you learn to fly and actually fly regularly, how do you know what kind of airplane will fit your needs?  2. Insurance - very low time pilots and kit planes aren't something insurers like. 3. Most pilots qualified to test fly the plane probably won't unless they know you and have seen the quality of your construction.  Have you read AC 90-89B?  I would suggest joining EAA and finding the local chapter.  They'll be glad to help guide you.

Other than your suggestions about not building a plane, which this question isn't about, why not give your perspective about the likelihood of pilots test flying the plane as an answer so people can vote on it? Thanks for the link, btw.

I'm not sure what answer you expect here: you've summed up the options fairly well and which one is best for your specific case depends on the aircraft and who's willing to fly it. As well as where you are, how much you're willing to pay etc. In other words, there's no clear answer here and if you want opinions and experiences then you might get a better response in a discussion forum. But FWIW, if you're an EAA member and you know your local chapter members, someone will almost certainly help out for free. If they know you well enough to trust your building skills, that is...

Contact the EAA.org - Experimental Aircraft Association. The EAA has volunteers world wide and information for all areas of experimental homebuilts - most of it is free.

Even licensed, experienced pilots often hire other pilots who have experience in test flying and hopefully time in the same model to perform the initial flights. Expect to pay for the service; test pilots do not do the work to build time. How hard it will be to find someone to do it depends on many factors such as the type of aircraft, the reputation of the design, your location and your willingness to allow the testing to take place at a safe field, the quality of your work, if you worked with an EAA chapter and at least one advisor, the quality of your build logs and photos, the quality of your flight test program docs, etc., etc. The EAA can further advise you on this, the membership fee is well worth it.

You don't need any kind of licensing for the pilot or the plane so long as you are flying an ultralight as defined under "FAA part 103."
If you Google "FAA 103" you can get the specifications straight from the source.  Some of the requirements are:
Weight of less than 254lbs empty. (Safety equipment does not count against this.)
Single seaters only.
Fuel tank no larger than 5 gallons.
Must be slower than 55 MPH in straight level flight.
Must have a stall speed slower than 24 MPH.
There are some other requirements that can't recall off hand.

This does not answer the question which is about experimental-amateur built (A-AB) aircraft.  E-AB has a required flight test period. This is unrelated to Part 103 ultralights.

This could be improved by providing a link to the FAA regulation and explicitly stating that you are answering only one half of the question.


text:
Can a holding pattern be in class G airspace?
Is separation provided between aircraft holding in class G airspace?
Could there be a holding stack in class G?

Which country or regulations are you asking about?

Pilots are told to hold in a holding pattern by ATC. Since they are under the control of ATC, by definition they are in controlled airspace. Class G is not controlled airspace so they couldn’t be required to hold in it.

By definition, Class G is uncontrolled airspace so the only separation you will receive there comes from using your Mk I eyeballs and good judgment.  A controller is not going to dispatch you to hold in uncontrolled airspace.


text:
Will manned aircraft have to defend ground targets against unmanned aircraft in the future?
Given that current Unmanned Combat Aerial Vehicle (UCAV) development is mainly directed towards the strike role, is it most probable that a future manned vs unmanned combat aircraft scenario will involve manned aircraft attempting to defend ground targets against attack by UCAVs?

Too bad this was put on hold, there was a 2008 incident between Georgia and Russia where this thing happened: a Russian jet took out a UAV.  But given that this is speculation about the future, I understand why it was closed as opinion based.

My understanding is that the UAV in question was unarmed.

It was, but had it been armed it would not have mattered for that engagement.

If the UAV had been armed, it might have destroyed the fighter. An unarmed UAV or UCAV could also destroy a manned fighter with a mid air collision.
In such a deliberate mid air collision, the manned fighter's nose radome and cockpit canopy would be the prime targets. Either would be destroyed by the impact of a small UCAV since they are only designed to cope with bird strikes.

If the UAV had been armed, it might have destroyed the fighter  Fighter has speed, altitude, and visual advantage.  per von Richtofen "whomever sees the other first will most often win the fight."  Air to air engagements are a little more complicated than you are giving them credit for.  An actual UCAV with the AI and algorithms that can address the complexity and decision cycle of an actual dog fight is another matter entirely ... that particular drone in 2008 was nowhere near that advanced. A UCAV or other drone needs a speed advantage to ram a fighter.

I don't think it's a good idea to generalise too much about air to air combat. There's a big difference between a low speed, low level scenario and a medium altitude scenario.

If we consider a low level air to air scenario of the kind that might arise during a strike mission by UCAVs against ground targets, the UCAVs could detect the fighter using sensors in the visual and infrared wavebands and then manoeuver to ram or to engage with  guns or missiles, depending on how they were armed. The UCAVs would probably have equality in speed and an advantage in terms of high g manouvering capability, at low altitude. My guess is that the UCAVs would detect the fighter first, because they would have a smaller signature individually.

Assuming a one on one engagement would be a big mistake, when the probability is that it would be more like 20 to 1 in favour of the UCAVs.

A UCAV does not need a speed advantage to destroy a manned fighter with a head on collision. During the Second World War, Japanese fighters sometimes used this tactic against the B29 Superfortress bomber.

A collision course interception actually requires very little in the way of processing capacity. The USAF were doing computer controlled interceptions using the SAGE system in the 1950s. This was before computers became small enough to fit inside fighter aircraft.

Anyone who has played Microsoft Combat Flight Simulator knows that a laptop is actually a very tough opponent in a dogfight.

Fundamentally, computers and robotic systems have it all over human beings when it comes to tasks requiring rapid decision making, fast reaction times, and high levels of coordination. This is the reality that air forces around the world are working hard to deny.

Some advantages of UCAVs over manned aircraft in the strike role:

The enemy doesn't gain hostages when they get shot down over enemy territory
The numbers that can be deployed are limited by the rate at which they can be turned out on automated production lines, not by the time it takes to train a combat fast jet pilot to a high standard (about 5 years minimum)
The UCAV can potentially be much smaller than a manned aircraft, if a reduction in range is acceptable, reducing the radar signature and vulnerability to air defence systems
As a result of 3, the UCAV can potentially be based nearer the area of operations and launched by catapult or using a rocket booster, leading to a shorter reaction time

In a scenerio where manned aircraft are intercepting UCAVs it is probably reasonable to assume that the UCAVs will have an advantage in numbers. UCAVs engaged in tactical missions will probably operate at low level and rely on superior manouverability and short range missiles to defend against attacks by manned fighters.

I'm not sure I understand why this was put on hold. The fact that that a UCAV doesn't need a pilot is just that, it's not a statement of opinion. All I've  really done is to draw a few logical inferences from that fact.

Basically, all arguments in favor of combat UAVs face the same problem - if it isn't fully autonomous, it can be incapacitated by disruption of control channel, and then it cannot use its advantage in speed/manouverability/numbers at all. If it is autonomous - we got ourselves a killer robot, what could possibly go wrong? /s And we cannot measure up these negatives against the positives at this moment due to us having no hard facts, only estimations of possible characteristics of UCAV. So the answer to your question is up to personal opinion, not to facts - thus it is closed.

Anyone who has any knowledge of military history knows that in war there are always a lot of things that can potentially go wrong, with or without robotic weapon systems.


text:
EASA Equivalent of CAA CAP 697 and CAP 698 Training Documents
I am wondering if by now there are EASA-issued documents containing the same information as the CAA examinations training documents CAP 697 ("Flight Planning Manual") and CAP 698 ("Aeroplane Performance Manual").
I did find that the EASA Easy Access Rules for Aircrew in AMC1 FCL.310; FCL.515(b); FCL.615(b); FCL.835(d) Theoretical knowledge examinations still mention that:

Excerpts from any aircraft manuals including but not limited to CAP 696, 697 and 698 for aeroplanes, and CAP 758 for helicopters may be used in training. Where questions refer to excerpts from aircraft manuals, the associated aircraft data will be provided in the examinations.

However, I could not find any equivalent training documents. Is the information spread out over multiple Easy Access rules documents?

CAP documents also referenced in Annex II to ED Decision 2016/008/R, and no alternative suggested.


text:
Historical Number of Aircraft in Service (by Airline)
I am looking for historical data on the number of commercial passenger aircraft in service, ideally with additional information on the associated airline.
My search so far has turned up one scientific publication (from Finnair maintenance) that provides data on the total number of aircraft, by type of aircraft:

Source: Kilpi, Jani. "Fleet composition of commercial jet aircraft 1952–2005: Developments in uniformity and scale." Journal of Air Transport Management 13.2 (2007): 81-89.
The data seems to have been sourced from the "ACAS" database, which was offered at the time by FlightGlobal:

ACAS is the leading aircraft database for Windows PCs. ACAS covers Western-built fixed-wing civil aircraft from 8 seats to ultralarge airliners. Also covers business jets and military transports. Click for a demo.

FlightGlobal has since spun out its data offering to Cirium, which offer data access at a hefty price tag. Whether their current offer also includes historical fleet fleet data going back to 1960 (or earlier) is not clear.
I found two other publications that might potentially be useful:

The well-known "JP Airline Fleets International" publication - seems to be available only in print, historical data would likely need to be extracted from a large number of editions.
The Air-Britain "Airline Fleets 2023" - again only in print.

ICAO and IATA have their own commercial data services, but those are (like the services of Cirium) rather expensive.
If there is any publicly available data, I would appreciate a hint on where to find it!

After some more investigation, I was able to find authoritative information - unfortunately, for US airlines only (and only for those that were members of the former Air Transport Association of America (ATA)):
The Past A4A Annual Reports (1937-2011) includes information both on total aircraft in service and on the respective types:

Source: 1964 ATA Annual Report: "50th Anniversary of Scheduled Air Transportation"

I've successfully obtained this information from two sources:
Annual financial reports from the airlines.  Aircraft are a major capital asset, they report how many they have each year.  I was able to download what I needed from the airline's website.
Hobbiest airplane spotter records.  These records were amazingly accurate.
In the case of Southwest airlines, the financial reports provided 1/year numbers of each kind of aircraft.
The airplane spotter records I found included a spreadsheet of every tail number, date it joined the fleet, date it left the fleet, etc.  I.e. 1/day resolution on the company's fleet.

Wikipedia has information for at least some airlines. I can't vouch for accuracy (which is probably not consistent, this being wikipedia after all).
Major question is how you'd consolidate the data into something useful. What are you going to do with airline mergers, splits, joint ventures, etc. etc.
For example KLM is fully owned by Air France, but fully owns Transavia and Martinair, as well as KLM Cityhopper. KLM USED TO have a joint venture with Northwest airlines for their transatlantic operations, how do you figure out which aircraft from each airline were committed to that and how to split them into the inventories of the partners? KLM owns (or owned) a percentage of Kenya Airlines, does that mean they in your numbers would own a percentage of their fleet?


text:
Is there a recommended self-study syllabus or set of books for UAV design?
I'm junior software engineer (C/C++/Assembly) in a UAV project. After working there for some months I decided that it would be very interesting to start my own project. So, my goal is to design my own UAV from scratch at home. The main problem is lack of knowledge: I don't know anything about aerodynamics, engines, mechanics and so on...
It's not a problem for me to find materials on the web. The difficulty is that I don't know which disciplines to take and in which order. I understand that you should be good at calculus, differential equations, solid mechanics, aerodynamics and so on.
So, the main question is: which books/resources would you recommend and in which order?
P.S. I have found some stuff on MIT OpenCourseWare but the information there is old and not complete. Books are much better. For example I found this one very helpful, but rather complicated.
Update: I decided to extend my question. So, let's try to structure all themes, that are needed.
Base:

Math.
Mechanics.
Dynamics/Kinematics
Aerodynamics.

Specific:

Engines.
Automatic Systems.

Comments are not for extended discussion; this conversation has been moved to chat.


text:
Is the in flight emergency manual of a Boeing 737 Max 8 available to read online?
I frequently read about pilots going through manuals/checklists/procedures  in an emergency situation in an airliner while in flight. Is there an online resource where the public can actually read it, for e.g. for Boeing 737 Max 8?

The in flight emergency manual is called the Quick Reference Handbook (QRH) and is in general not made publicly available by the manufacturer or airlines.
It is however possible to find quite a few online. Pages like smartcockpit.com have a lot of resources available. The Boeing 737 MAX is not mentioned on their page. A Google search for "737 max qrh" may help you find something.


text:
What effect do slats have on descent rate?
During descent, assuming the same speed and idle thrust, is the rate of descent greater in a clean configuration or with slats?

Any high lift device is going to decrease your l/d ratio. So for the same forward speed you will have a higher descent rate.

how can you demonstrate that? Even for slats, the lift to drag ratio will decrease in comparison with clean configuration, for the same speed?

The whole point of high lift devices is to maintain lift at lower speeds. Same lift + lower speed = higher induced drag. To maintain the same airspeed with slats/flaps deployed you have to pitch down, thus increasing descent rate (assumuing idle thrust).

Clean config will be designed to give you the best l/d since that’s what you want in cruise: highest possible speed at the lowest possible thrust setting.

So, in your opinion
l/d with slats is < l/d in clean config.
Do you have a graphic or some publication that can prove it ?

Yes. Flaps increase l/d even more. That’s what they’re for, to increase lift at lower speeds. The wing is designed for the best l/d, but at low speed they won’t have enough lift to overcome gravity, so you use slats/flaps to increase lift at the same speed. This will increase induced drag. Slats have less effect on the l/d than flaps. That’s why they are used on takeoff and deployed first during descent. If they didn’t decrease l/d than they would design them to be used at cruise speeds.

I’m hoping someone will answer that has the maths for all this. I’m just going on generalities. But the wing will always give you the best l/d in clean config. Thus why you don’t deploy devices to get best glide

I agree with you all the way, just trying to find some scientific explanation. thanks for your time

This question explains the effects of slats/flaps on l/d ratio

If the slats would increase L/D, they would not be retracted in cruise.
TomMcW is right, any high lift device increases drag when compared to a polar point that is within the regular operating envelope of the clean wing. How much so can be answered when looking at old wind tunnel data published by Sighardt Hörner in his classic "Fluid Dynamic Drag":

Figure 6-20 from Fluid Dynamic Drag comparing different versions of the Fieseler Fi-156 "Storch" slat. Although it is not explicitly stated (and the original report in the Jahrbuch der deutschen Luftfahrt-Forschung of 1940 is not available online), it must be assumed that the lift coefficients of (a), (b) and (c) are similar. After all, Mr Hörner worked at Fieseler in Kassel and contributed to the aerodynamic design of the Fi-156, so he should have been intimately familiar with this airfoil.
Version (a) at the bottom is with a gapless slat and produces a drag coefficient of 0.015. Version (b) is with a gapped slat but blocked flow, and the drag coefficient shows a small increase to 0.018. Once the gap is opened (c), the drag coefficient jumps to 0.03. Note that at a higher angle of attack and lift coefficient (d), the drag coefficient is slightly reduced (0.028). But it is safe to assume that a deployed slat roughly doubles the airfoil drag.
If thrust and speed are not changed, deploying slats will increase sink speed. If I further assume that this descent is done at optimum L/D (zero-lift drag equals induced drag), then a doubling of the zero-lift drag of the wing will increase total drag by maybe 17%, details depending on the share the wing has in overall drag (I assume for the figure above that this share is ⅓).


text:
How can I find out the reason for an aborted takeoff?
I was listening to ATC at LAX today and heard them cancel AA 253's takeoff clearance and instruct them to abort their takeoff; it sounds like they were already rolling down the runway when it happened. The call to stop was pretty insistent.
In a lot of hours of listening, I don't remember hearing that happen before, I'm really curious why they did it. Is there any good way to find out?

You can rewind audio at liveatc.net. If that is what you mean.

It may get reported on the Aviation Herald, but not all do so there is no guarantee. It not there yet. http://avherald.com

There's a bunch of YouTube channels dedicated to  "live ATC" recordings. They usually combine ATC from various frequencies (e.g. approach, tower, ground) and tracking websites to create the full picture. If you're lucky your occurrence has a video.

An instruction to abort takeoff by ATC to a single aircraft is almost always due traffic in its widest sense - conflicting airborne or ground traffic, runway occupancy, separation requirements and the likes. It is likely possible to find out if the traffic situation can be reconstructed (see answer on how to start below by @nabla). Had the flight crew called an abort without giving a reason, it would be much harder to know any reasons.

In addition to listening to ATC audio recordings as suggested by previous answers or comments, you can also try to visualize the traffic situation at the time of the event using the playback feature of flight tracking websites like FlightRadar24.
On those radar images, you can possibly see other aircraft, whether there was the danger of an imminent runway incursion, maybe the go-around of another aircraft nearby or something similar which prompted ATC to cancel the takeoff clearance. If you combine those radar playbacks with the audio recordings that you already know, you might get a pretty accurate picture of what actually happened.


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What is the exact definition of Taxi Time?
I found some definitions on the web, but they weren't same.
Some resources mentioned it is sum of the times the aircraft is moving on the ground with its own engines. Some defined it as the time between chocks-on and take-off.

Definitions always depend on the context. Taxi time may be defined differently in SESAR, NextGen, Airline operations, aircraft maintenance and CDM.

VTC as too broad for the reason mentioned by @DeltaLima above

For what it may be worth, in my experience in light aircraft back in the 1980s and before we didn't keep track of taxi time. The logged time and the time we paid for was taken from the Hobbs meter, however it was hooked up. In the 1990s on large aircraft, the flight engineer recorded the  first movement of the aircraft as the start of taxi, whether that movement was a push-back or powered. He also recorded the the off and on time and the setting of the brake at parking. The pay records supplied to us by the company showed these times, and most of us just kept those instead of actually logging.

As always, the definition depends on the context. That is why official documents always included the definitions of important terms used in the text.
For Collaborative Decision Making (CDM) as defined  by EUROCONTROL in the latest CMD manual (2017), taxi time is divided in taxi-out time and taxi-in time.

3.4.2 Definition of Taxi Time
For Airport CDM purposes, taxi time is considered to  be:

For arriving flights: the Actual taXi-In Time (AXIT)  is  the  period  between  the  Actual  Landing  Time   (ALDT) and the Actual
In-Block Time (AIBT)
For  departing  flights:  the  Actual  taXi-Out Time   (AXOT) is the period between the Actual Off-Block Time (AOBT) and the Actual
Take Off Time (ATOT).

For  calculation  purposes  within  the  CDM  Platform, taxi times
will be referred to as estimated taxi-in (EXIT) and estimated taxi-out
(EXOT) as there is no requirement for a scheduled, actual or target
taxi time.

Now that in itself doesn't help if you don't know how the ALDT, AIBT, AOBT and ATOT are defined.
The ALDT is the same as the ATC Actual Time of Arrival, which is the time the aircraft touches down.
The AIBT is the actual time when the parking brakes have been engaged at the parking position. (source)
The AOBT is the time the aircraft pushes back / vacates the parking position. (source)
The ATOT is the same as the ATC Actual Time of Departure, which is the time the aircraft lifts of from the runway.
Now Taxi time defined as above is used in Airport management, Air Traffic Management. It may be different in other areas.
Some observations that I want to make given the above definition:

De-icing on-stand is excluded from the taxi-time. De-icing on a remote de-icing pad is included in the taxi-time.
The take-off run is included in the taxi-time.
The landing run is included in the taxi-time.
Taxiing between stands or to/from maintenance is not defined.

Time from the start of motion of an aircraft, under engine power, until the cessation of motion at the completion of a flight, minus flight time.
Addendum #1
Block time, an industry term in the US, refers to the time the aircraft departs the blocks initiating a flight, until it returns to the blocks at the completion of the flight.
Block time may be dichotomized into flight time and taxi time.
Flight time strict definitions vary, but the general industry term is from the initiation of the take off roll, to the stopping of the aircraft after landing, or the departure of the aircraft from the landing runway, which ever comes first.
Taxi time is the time the aircraft spends in movement or holds on the surface of the airport, prior and subsequent to flight.
While the above is generally practiced industry definitions, there are notable variations, as there is no absolute terminology agreement between the various industry segments in aviation and the regulatory segments.
For example, in the US, 14 CFR 1.1 provides a definition of "flight time" which is analogous to "block time" in the industry.  However, in Europe, JAR 1.1 defines "flight time" as airborne time. Even that definition varies from the industry common practice of the time from initiation of take off roll until the stopping of the aircraft or clearing the runway.
In the US, block time (aka 14 CFR 1.1 "flight time") is normally logged for pilot experience purposes, and includes taxi time associated with movement of the aircraft to/from the runway and the ramp.
ICAO Annex 1 is slightly different from the US FAA definition.
Since the OP asked for a definition of taxi time, I answered this question with the generally, although not 100% universally, industry defacto terms.

Do you have a source for that definition? At least in the US, "flight time" includes taxiing.

From the Pilot Controller Glossary

TAXI− The movement of an airplane under its own power on the surface
  of an airport (14 CFR Section 135.100 [Note]). Also, it describes the
  surface movement of helicopters equipped with wheels.

§135.100   Flight crewmember duties.

(c) For the purposes of this section, critical phases of flight
  includes all ground operations involving taxi, takeoff and landing,
  and all other flight operations conducted below 10,000 feet, except
  cruise flight.
Note: Taxi is defined as “movement of an airplane under its own power
  on the surface of an airport.”

The above cited references define the function "taxi" not "taxi time."

@mongo If taxiing is “movement on the surface”, then it logically follows that the time you spend moving on the surface is taxi time.


text:
How can I convert a frozen ATPL from South Africa to an Iranian one?
I have got a frozen ATPL license from South Africa and I'm from Iran. I came back home last year and I have converted my CPL/IR license to the Iranian piloting license by doing Air Law and Human Performance exams plus a skill test.
I'm willing to get the frozen ATPL license in Iran but I don't know what the rule is and if I can get it without doing the ATPL exams again.
I checked Annex 1 (Eleventh edition) but there wasn't anything about it.
I assume both countries are following ICAO rules so I would appreciate it if someone helps me to know where or what the rule is.

This is a very specific question and I don't know if anyone here will be able to answer it. If not, I would simply ask the Iranian authorities about it. You said you've already converted your SA license to an Iranian one, so I guess you've had some contact with them already. And if you do find out the answer, please feel free to come back here and answer your own question!

And you might want to clarify what you mean by a "frozen ATPL".

@JuanJimenez I mean I have done all ATPL exams but I don't have 1500 hours so It's still frozen . So I would like to know If I have to do the exams in Iran again or not ?!

@JuanJimenez You're not the only person who wondered about that :-)

I'm thinking you should talk to an aviation school that specializes in training for Iranian students, like http://www.paaviation.com or this one  www.apts.ir/en .


text:
What is the effect of airflow speed on separation?

We know that in real viscous flow, boundary layer exists around the airfoil and there will be a separation point which flow starts to separate from the back of the airfoil( velocity gradient equal zero at separation point). If the flow speed increases, will separation occur earlier or later?

Is this a homework question of some sort, or otherwise assigned to you?

@RalphJ Nope just curious about it

@mins yeah that's exactly what I mean

In a subsonic flow, the separation point moves downstream as Reynolds number increases, and Reynolds number increases with free stream velocity.
Demonstration on a cylinder-shaped airfoil:

Source

From Wikipedia:

Reynolds number is the ratio of inertial forces to viscous forces
  within a fluid which is subjected to relative internal movement due to
  different fluid velocities

Separation occurs at the location the laminar airflow stops being accelerated, and is decelerated by areas of upstream turbulent airflow in separation bubbles created by friction (viscosity):

Source
Velocity gradients in the boundary layer at different points in the transition area where the flow transitions from laminar to turbulent:

Source, adapted
Velocity is decreasing near the surface due to friction, and at some point becomes negative. Separation bubbles appear between the laminar flow and the airfoil. In a bubble there is a secondary reverse flow and a region where air is stagnant. Downstream the bubble, air is turbulent:

Source, adapted
While other elements affect the boundary layer separation point, this one is the largest. To counter this effect, the momentum of the laminar stream must be increased relatively to frictional forces. This is done (by definition) by increasing the Reynolds number, e.g. by increasing the airspeed.
Increasing airspeed can be obtained by accelerating the aircraft, but also by other ways. For example, different devices are used to delay the separation of the boundary layer when the aircraft must be slowed down. It consists on accelerating ("re-energizing") "dead air" near the surface, e.g. using high speed jets or vortex generators.
This is for instance the principle behind slats and slotted flaps:

Source, adapted

The boundary layer separation is due to flow deceleration as it moves away from the leading edge and an adverse pressure gradient appears (potential flow theory).
The lower energy flow is more prone to perturbations, vortexes appear and disappear, increasing in importance. Portions of vortexes have a velocity opposed to the free stream velocity, this creates areas of "dead air". Viscosity takes over inertia. This ratio between inertia and viscosity effects is the very definition of the Reynolds number.
By increasing velocity (actually the Reynolds number), this effect is minimized: The separation point moves downstream, as the effects of the adverse pressure gradient are countered on a longer distance as visible on the velocity gradients shown on the flap/slat picture above.
Flaps/slats have the additional advantage (over vortex generators for instance) of increasing the wing surface, hence restoring lift lost when the airplane was slowed down.

Thank u very much. This answer is exactly what I want. Initially, I thought in this way: higher speed means more time to decelerate the velocity gradient to 0( where separation occurs). So the separation point moves downwards. It seems too simple to be wrong. Could u explain more about why separation point move downward and why related to Reynolds number directly?

@HanzhiZhang: At higher speed the air molecules come with more inertial energy, but viscosity has not changed. This means more "oomph" is available to be eaten up in friction before the air comes to a standstill relative to the wing.

@PeterKämpf thanks I understand it

What a wonderful answer!

@PeterKämpf,what is "oomph" ?

@mins,isnt flow separation happend because of adwerse pressure gradient not friciton(viscosity)?

@AeronauticFreek Without the slowing down from friction the adverse pressure gradient would not be enough to bring the air to a complete stop. Friction is essential because it robs the boundary layer of energy which is then missing when the pressure gradient adds another deceleration.

@AeronauticFreek: Friction at the wing interface, propagated partially to the whole boundary layer by air viscosity, creates a continuous decrease of airspeed along the chord, hence a deceleration of the flow. In the boundary layer, along the chord and at any height, there is a continuous increase of static pressure, and a decrease of dynamic pressure. This variation of static pressure along the chord is the gradient (∇ variation). So everything starts with the initial friction on the wing, low velocity will be seen in the inner boundary layer, and in the rest of the layer, at some degree.

@mins,@PeterKampf, are you also members at physicsforum? my opinion is that stackechange have great people for aerodynamics,but here is forbiden  to comment to much like on real forum,so what are your suggestion where is best forum for aerodynmics where we can deeply talk?

@AeronauticFreek: You need to use the chat (Hangar for broadcast)


text:
How to calculate outside air temperature at a certain altitude?
I am a PPL student and working on the cross-country planning.
Let's say the OAT is -13º @ 6000', how to calculate the OAT @ 7000'?

The temperature will drop around 2° Celsius (-1.98 to be more precise) for each 1000 feet you climb.
So if the OAT at 6000ft is -13°C, at 7000ft it will be -15°C, or minus 16 from the International Standard Atmosphere (ISA) which should be +1°C (15 - 2*7 = 1).

While the ISA lapse rate, normally 3.5F or 2C per 1000 ft is a good ISA approximation, one must keep in mind that fronts, inversions and other weather related phenomena may alter temperatures from the normal lapse rates.
If you are attempting to make accurate estimates of temperatures, while you can apply standard lapse rates, I would encourage you to examine winds aloft, in necessary interpolating the temperatures between forecast points. Then one would interpolate using the forecast winds aloft temperatures.
Looking at a local winds aloft forecast, I see:
3000 2914-05
6000 2820-06
9000 2727-17
So from 3000 to 6000 the forecast is much less than the lapse rate, but from 6000 to 9000, the forecast is greater than the standard lapse rate of 2C/1000ft * 3000ft = 6C but the forecast difference is 11C.  So if one were to estimate the temperature at 8000 for this area, I would use:
-06C for 6000, minus 2000ft/3000ft * 11C = -6C - 7.33C or about -13.3C
In contrast with the standard lapse rate, in this instance the calculation would be:
-06C - (2 * 2) = 10C
Sometimes the forecast values are substantially different from the standard lapse rate. If temperature were important, I would want to develop the best estimate and use the forecast temps aloft.


text:
What is the difference between an RPL and a PPL licence?
I am looking forward to starting my training to get my RPL license and I was wondering what the difference was because on the website it looked like you have to do 2 different pieces of training for each license. If I was to get my RPL in Australia what would the difference be between the two?
Thanks,

An RPL requires less training (i.e. money) but is more restricted in how far you can fly, how many passengers you can carry and how heavy the aircraft can be. You can get an endorsement to fly anywhere domestically; there is no endorsement available to fly internationally or for the other two restrictions, nor is an instrument rating available.
A PPL allows you to fly just about any small aircraft anywhere, and you can add an instrument rating as well if desired. There is still a weight limit, but it's high enough you wouldn't hit it without needing to get (at least) a commercial license anyway.
All of the training you take for an RPL will count toward a PPL, so if you start on the former and switch to the latter, you lose nothing. Or you might get an RPL first and, if you find the it too limiting, upgrade to a PPL later; the only thing you lose in that case is having to pay for twice as many tests.


text:
Is DAL-A required for twin-engine helicopters with FADEC?
When operating Twin Engine helicopter with FADEC controls, is is necessary to certify at DAL-A?  If so, why? Is it related to the FADEC certification or is this required for an aircraft potentially roling into a military platform?

That is going to depend on the System Safety Analysis for the aircraft

Thanks... I was inquiring of that earlier, likely candidate for this discussion is data acquisition only, not likely requiring Class A

Wait, are you talking about the FADEC software DAL level or something else? I would expect the FADEC software to be either DAL A or B, depending on the installation and system safety analysis.

data acquisition related to FADEC, but not part of the FADEC

With no reversion to a direct mode, ie retaining control of the engine possibly with limitation after fadec failure would imply DAL A since that fault results in loss of ability to maintain flight.
With some form of control reversion, the authorities would accept a much lower dal.
You cannot claim credit for the second engine since both engines use common. Software and hardware. So any common cause, common mode fault could take out both engines.
For part 29 helicopters  I would expect Dal A, part 27 probably B..

One thing that must be addressed is that while the engine is certificated as part of the airplane or rotorcraft, it first must receive its own Type certificate under Part 33. The requirements there often are more stringent than at the aircraft level as the engine is normally developed for the most stringent application of the engine.
Looking for guidance we can start with Part 33 which gives us the rule that the DAL must be appropriate to the risks identified in the safety analysis. Since the question is about FADECs, it's worth looking at the Advisory Circulars, starting with AC 33.28-3 Guidance Material For 14 CFR §33.28, Engine Control Systems.
Chapter 9 addresses software.

9-1. Rule Text. Section 33.28(g) reads:
“(g) Software. The applicant must design, implement, and verify all
associated software to minimize the existence of errors by using a
method, approved by the FAA, consistent with the criticality of the
performed functions.”

Moving down to the software section:

9-2. Guidance: Software.
d. Software levels.
(1) The level of software required for certification depends on the
criticality of the functions it performs. For example, failures
resulting in significant thrust or power increases or oscillations may
be more severe than an engine shutdown. Therefore, consider these
failures when selecting a given software level.
(a) Design, implementation, and verification of software as specified
in Level A (DO-178C) is normally needed for turbine engines.
(b) For a reciprocating engine EECS, software implemented as specified
in Level C is the minimum acceptable requirement.
(c) The applicant may choose to evaluate the failure condition
criticality of EECS functions to determine if Level B or C software
would be adequate. The applicant must coordinate this evaluation with
the aircraft designer and the cognizant aircraft ACO during the EECS
development program.

So it would appear that assuming your twin-engine helicopter used turbine engines, the FADEC would require Level A software.